The present invention relates to a hydraulic control system for controlling the width of a drive pulley and a driven pulley in a belt-type continuous variable transmission, more particularly relates to the hydraulic control system provided with valves for respectively directly controlling oil pressure for controlling the width of the drive pulley and the driven pulley.
A belt-type continuous variable transmission (in other words, a continuous variable V-belt transmission) comprising a pulley width variable drive pulley, a pulley width variable driven pulley and a belt member wound between these drive pulley and driven pulley is already well-known and is practically used. The above transmission is provided with a drive side hydraulic actuator for controlling the width of the drive pulley and a driven side hydraulic actuator for controlling the width of the driven pulley, controls the setting of pulley width by controlling oil pressure supplied to both hydraulic actuators and can variably set transmission gear ratio steplessly and continuously.
For such a system for controlling oil pressure supplied to a hydraulic actuator, there is a system using a four-way valve disclosed in Japanese Patent Unexamined Publication No. Hei. 8-219188 based on an application which is filed by this applicant into Japanese Patent Office. FIG. 7 shows a control system using such a four-way valve and the control system will be briefly described below. First, a continuous variable transmission is constituted so that transmission control is executed by controlling lateral pressure which acts upon a movable pulley half 13 of a drive pulley 11 based upon oil pressure in a drive side cylinder chamber 14 and also controlling lateral pressure which acts upon a movable pulley half 18 of a driven pulley 16 based upon oil pressure in a driven side cylinder chamber 19.
The above hydraulic control system controls oil pressure supplied to these drive and driven side cylinder chambers 14 and 19, produces line pressure PL by regulating the oil pressure of hydraulic fluid in a tank T supplied from a pump P using regulator valves 81 and 82 and produces modulator pressure PM by decompressing the line pressure using a modulator valve 86.
The line pressure PL is supplied to a high/low-pressure control valve 83, control back pressure corresponding to control over the energizing of its linear solenoid 83a is regulated using the high/low-pressure control valve 83 and supplied to a low-pressure regulator valve 84. Line pressure PL is supplied to the low-pressure regulator valve 84, is regulated corresponding to control back pressure and low control pressure is supplied to right and left inlet ports of a shift valve 85 comprised of a four-way valve via a line 102. In the meantime, line pressure PL is supplied from a line 101 as it is to the inlet port in the center of the shift valve 85 and these oil pressures (line pressure PL and low control pressure) are respectively distributed and supplied to the drive side and the driven side cylinder chambers 14 and 19 via lines 103 and 104 by the operation of the shift valve 85.
In the meantime, modulator pressure PM is supplied to a shift control valve 87 via a line 105, the shift control valve 87 produces shift control oil pressure according to control over energizing a linear solenoid 87a and supplies the shift control oil pressure to the shift valve 85 via a line 106. As shown in FIG. 7, the shift valve 85 is provided with a spool 85a pressed left by a spring 85b and control oil pressure applies rightward pressure to the spool 85a. Therefore, the position of the spool 85a can be controlled by controlling the magnitude of control oil pressure and hereby, transmission control is executed by controlling respectively distributing line pressure PL from a central inlet port and low control pressure from the right and left inlet ports from the lines 103 and 105.
Line pressure and low control pressure controlled so that line pressure and low control pressure are correctly distributed as described above mean oil pressure provided with fixed differential pressure determined by the low-pressure regulator valve 84 and the magnitude of both is arbitrarily set by the high/low-pressure control valve 83.
A hydraulic control system using separate valves for respectively directly controlling oil pressure supplied to the drive and driven side hydraulic actuators is also proposed as in Japanese Patent Examined Publication No. Hei. 6-74839 for example.
This publication discloses such a hydraulic control system shown in FIG. 8 and referring to FIG. 8, the hydraulic control system will be described briefly below. As the configuration of a continuous variable transmission to be controlled is the same as that shown in FIG. 7, the same reference number is allocated and the description is omitted. In the hydraulic control system, the oil pressure of hydraulic fluid in a tank T supplied from a pump P is regulated by regulator valves 91 and 92, line pressure PL is produced and modulator pressure PM is produced by decompressing the line pressure by a modulator valve 93.
The modulator pressure PM is supplied to first and second linear solenoid valves 94 and 96 respectively via lines 111a and 111b and desired first and second control back pressures PB1 and PB2 are respectively supplied to lines 112a and 112b by controlling the energizing of linear solenoids 94a and 96a. The above first and second control back pressures PB1 and PB2 are respectively supplied to first and second pulley control valves 95 and 97 as shown in FIG. 8, these pulley control valves 95 and 97 regulate line pressure PL supplied via a line 114 and produce first and second control oil pressure PC1 and PC2 respectively corresponding to the first and second control back pressure PB1 and PB2. The first and second control oil pressures PC1 and PC2 produced as described above are respectively supplied to the drive side and driven side cylinder chambers 14 and 19 via lines 113a and 113b.
As described above, in the hydraulic control system, transmission control is executed by controlling the first and second control oil pressure PC1 and PC2 respectively supplied to the drive and driven side cylinder chambers 14 and 19 by controlling the energizing of the linear solenoids 94a and 96a and controlling varying the width of the drive pulley and the driven pulley.
In the hydraulic control system, a third linear solenoid valve 98 for receiving modulator pressure PM via a line 111c and producing third control back pressure according to the energizing of a linear solenoid 98a is further provided. The third control back pressure is supplied to the regulator valve 92 via a line 115 and line pressure PL regulated and set by the regulator valves 91 and 92 can be arbitrarily controlled.
As described above, heretofore, various hydraulic control systems are proposed, however, first, in the case of the hydraulic control system shown in FIG. 7, as line pressure PL is provided with fixed differential pressure from low control oil pressure, there is a problem that excessive differential pressure causes the useless work of the pump P in an area in which the differential pressure of control oil pressure supplied to the drive and driven side cylinder chambers 14 and 19 is not required so much, and the deterioration of fuel consumption and the rise of oil temperature may be caused.
Further, as line pressure PL is provided with fixed differential pressure from low control oil pressure, there is a problem that transmission speed is restricted by differential pressure set as described above and if transmission is to be sped up, transmission speed cannot be set to speed exceeding speed-based upon the differential pressure. The more the differential pressure is, the faster transmission speed can be, however, if differential pressure is increased, a problem of the deterioration of fuel consumption and the rise of oil temperature when differential pressure is not required becomes more remarkable.
In the meantime, in the case of the hydraulic control system shown in FIG. 8, as line pressure PL can be independently controlled by the third linear solenoid valve 98, it is considered that the above problems can be solved. However, in the system, the control system is required to be constituted using the total three solenoid valves of the first to third linear solenoid valves 94, 96 and 98 and there is a problem that the configuration of the control system is complicated and the cost is increased. Further, these three solenoid valves are required to be independently controlled and there is also a problem that control is difficult (a load upon control software is increased).
In the above Japanese Patent Examined Publication No. Hei. 6-74839, an example in which line pressure is set based upon the control oil pressure of the higher of control oil pressure respectively supplied to the drive and driven cylinder chambers is disclosed. In this case, there is an advantage that line pressure can be also set only by the solenoid valve for regulating control oil pressure supplied to the drive and driven side cylinder chambers. However, as in the system, line pressure is set using control oil pressure itself supplied to the drive and driven side cylinder chambers respectively provided with large volume, there is a problem that the rise of oil pressure is delayed in a process for supplying and filling oil to the cylinder chamber for example when speed is varied (particularly speed is rapidly varied) and the rise of line pressure is also delayed, that is, line pressure control delay may be caused.
Further, in case the volume of the drive and driven side cylinder chambers is different, the control oil pressure of the higher side is different between thecaseofup-shift speed varying control and the case of down-shift speed varying control and the characteristic of delay in the rise of oil pressure when speed is varied is also different. Therefore, there is a problem that the characteristic of the rise of line pressure may be different and control speed may be different between up-shift and downshift.